The present invention relates to a flexible buss bar having selectively clad solder pads attached at predetermined intervals along the length thereof such that the buss bar may be electrically bonded to electrical structures disposed on a substantially flat substrate such as a glass pane.
In many of today's automobiles the rear glass window pane includes electrical circuitry adhered to the surface of the glass or embedded within the glass pane itself. Typically this circuitry will include an electric rear window defroster, and possibly power leads to supply the center rear brake lamp, or some other circuitry. A typical rear defrost system is shown in FIG. 1, including a glass pane 10, power rails 12, 14, and a plurality of heating elements 16 extending between the power rails 12, 14 across the rear viewing area of the window. The circuitry shown in FIG. 1 also includes power leads 18, 20 extending from each power rail 12, 14 to the rear center brake lamp 22. In most cases this circuitry is formed of a silver ceramic material and is adhered to a surface of the glass pane through a silk screening and firing process. The firing process sets the silver ceramic material resulting in a hardened pattern of conductive traces firmly embedded on the surface of the glass pane. However, because the traces are formed of a combination of silver and ceramic, the traces are not purely conductive. Each trace will have an associated resistance which is determined by the percentage of silver included in the silver ceramic mixture, and the cross sectional area of the trace itself. The thickness of each component is generally uniform throughout, such that the actual resistance of each component is determined by the width of the silver ceramic trace comprising the component. It follows then, that the narrow heating elements 16 will have greater resistance than the wider power rails 12, 14. When a voltage differential is applied across the two power rails 12, 14, an electrical current flows through the heating elements 16 from the higher potential rail, for example power rail 12, to the lower potential rail, for example power rail 14. Due to the generally higher resistance of the heating elements 16, the electrical current through the heating elements will generate heat sufficient to melt ice and evaporate condensation which may have built up on the surface of the window.
In order to power the electrical defrost unit depicted in FIG. 1, it is necessary to form an electrical connection between the power rails 12, 14 and external drive circuitry which is connected to a power source located elsewhere on the vehicle. The process of attaching external circuitry to the embedded circuitry on the glass presents a number of manufacturing problems. First, it is difficult to provide a suitably strong bond between the external circuitry and the embedded circuitry sufficient to ensure a reliable and durable connection. Furthermore, the prior art solutions to this problem have been relatively expensive both in to terms of the materials necessary to form the electrical connections, and the labor involved in making the connections.
To date, one of two different methods have been typically employed in bonding external circuitry to circuitry embedded within an automobile rear window pane. Both methods involve soldering an external electrical connector directly to the electrical power rails 12, 14. The first method involves soldering a single metallic clip, usually formed of copper, directly to each of the silver ceramic power rails 12, 14. A commonly employed geometry for such a metallic clip is that of a T. With this design the horizontal arms of the T are pre-clad in a solderable material and the vertical leg of the T forms a tab to which an external wire is attached. The pre-clad solderable material of a first clip is re-flowed to bond the clip to the first power rail 12, and the pre-clad solderable material of a second clip is re-flowed to bond the second clip to the second power rail 14. External wires are then attached to the vertical portion of the T shaped clips and connected to a wire harness which electrically connects the two power rails 12, 14 to a power supply.
In the second method of attaching external circuitry to the embedded circuitry, a flexible buss bar is provided having a plurality of preformed solder buttons attached thereto. Typically the flexible conductor is formed of a multi-stranded braided conductor to provide both strength and additional flexibility to the external buss bar. The solder buttons are located at intervals along the buss to provide multiple contact points with the power rails 12, 14. With this technique, the flexible buss is placed against the glass pane with the various solder buttons aligned against the power rails, and the solder buttons are re-flowed to bond with the silver ceramic material comprising the power rails. Re-flowing the solder buttons has generally been accomplished by manually applying a soldering iron to each individual solder button, a time consuming and labor intensive operation.
Both of the methods outlined above present unique difficulties in the manufacturing process of automobile rear window assemblies. In the case of the single clip attach method, larger quantities of silver are required when forming the power rails 12, 14. Because there is only a single attachment point, electrical current must be carried the entire length of the power rails to drive the heating elements located furthest away from the point of attachment. Near the point of attachment, the buss must be capable of carrying all of the current necessary to drive all of the circuitry embedded on the glass pane 10. Thus, an extra wide buss is required in order to accommodate these large currents. A wider buss requires greater amounts of silver and increases the material costs in manufacturing the window assembly. Furthermore, the single connection method results in a circuit with no redundant connection in case the primary connection fails. If this single connection fails, all of the electrical devices included on the window will become inoperable.
The solder button technique avoids these problems, but generates manufacturing difficulties of its own. This technique, employing an elongated braided conductor specifically adapted for the purpose of connecting to electrical components embedded within a glass pane is disclosed in U.S. Pat. No. 5,143,273 assigned to Methode Electronics Co. and is incorporated herein by reference. The braided conductor disclosed in the U.S. Pat. No. 5,143,273 includes a plurality of solder buttons attached at regular intervals along the length of the braided conductor. The solder buttons each comprise a mass of solder surrounding the braided conductor and enclosing a small amount of solder flux. When attaching the braided conductor to the glass pane, the solder buttons are placed against the silver ceramic buss power rails 12, 14 and heat is applied to the solder buttons, causing the solder buttons to melt against the rails. When the solder buttons cool, the solder re-solidifies and forms a strong reliable joint between the silver ceramic rails and the braided conductor. The advantage of this method is that, because there are multiple contact points, only a fraction of the total current consumed by the embedded circuitry actually flows through the power rails at any given point along the length of each rail. Thus, the buss can be made much narrower, thereby reducing the material cost of the silver power rails. This method has the further advantage of including multiple contact points with each rail such that if one solder joint fails, the full current may still be provided through the remaining contact points. Although the above described process results in a strong and reliable connection between the external circuitry and the embedded circuitry, the process is cumbersome and time consuming, and does not lend itself to automation. Furthermore, in re-flowing the solder buttons manually, a large amount of variability is introduced into the bonding process. Thus, the quality of the connections can vary significantly from one connection to the next, as well as from window assembly to window assembly. Such variability is highly undesirable in a mass production environment where consistent, high quality, and reliable connections must be repeated with every operation.
In light of the drawbacks contained in the prior art, what is needed is an improved flexible buss bar which lends itself to a wider range of attachment methods. It is desirable that such an improved buss bar would include contact pads attached to the buss bar at regular intervals along the length of the buss. The contact pads should be pre-clad with a solderable material such that when placed against contact structures formed on the glass pane, for example pre-screened silver ceramic power rails, the preclad material can be re-flowed in a controlled manner to form a strong reliable joint between the flexible buss bar and the electrical circuitry carried by the glass pane. Additionally, the improved flexible buss bar should be configured such that it can be robotically positioned against the glass and a precision soldering method employed to re-flow the solderable cladding surrounding the contact pads. Thus, an improved flexible buss bar would allow a highly automated process for bonding the buss bar to a glass pane wherein the quality of the individual connection points may be rigidly controlled, and accurately repeated over numerous soldering operations.